Emergency lighting has been employed for several decades, for example to provide power to one or more light sources for illumination of the path of egress from a building or facility. Emergency lighting is required in industrial, commercial, and institutional buildings as part of the safety equipment. Emergency lighting relies on a limited backup power source for example a battery, to supply power to the light source(s). Here, “a battery” is understood to encompass a plurality of individual battery units connected together to function as one backup power source. An emergency lighting system (sometimes referred to as an “emergency ballast”) is designed to energize the light source(s) exclusively during periods of AC power failure, when the emergency lighting system is said to be in “emergency mode” (EM), and may be combined with a conventional lighting unit (sometimes referred to as an “AC ballast”). The emergency lighting system may sense the absence of the AC power and use the backup power source and dedicated electronic circuitry to energize the light source(s) during a limited period of AC power failure. In the USA, the required emergency lighting period is at least 90 minutes, while in Europe, e.g., it is 180 minutes, during which the emergency illumination level should not decline to under 60% of the initial level, as set for battery-powered emergency lighting systems by the life safety codes (e.g., section 7.9.2 of NFPA-101 and NEC 700.12).
Recently, light-emitting diodes (LEDs) have become more prominent in the market as a main light source for an occupied space. LEDs offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. These advantages are leading to the introduction of LEDs into a wide variety of applications and context. In particular, LED light sources are now being developed for use in emergency lighting systems.
In order to be able to provide the necessary amount of energy during the EM discharge, the battery is customarily charged for a period of time of at least 24 h, during which it should accept charge at an optimum rate no matter what temperatures (within a specified range) the emergency lighting system is subjected to. Especially for emergency lighting systems designed to operate at ambient temperatures below 0° C. (e.g., unheated garages, outside staircases, etc.), a means to warm up the batteries—generally only necessary during the charging phase—may be provided. Thus, the normal 0° C. low end ambient temperature limit of the emergency lighting system may be extended to −20° C. or less, without exposing the battery to the threat of explosion due to a potential build-up of hydrogen gases—which is known to occur with the typically used nickel-cadmium (NiCd) rechargeable batteries if full-charging is attempted while the batteries are frozen.
Furthermore, it is desired to be able to install a particular emergency lighting system in a variety of locations and to be able to connect it to a variety of different commonly encountered nominal AC voltages, for example 120 VAC, 230 VAC, 277 VAC, etc. However, accommodating such a wide range of possible input voltages can present a problem for the battery heater because the power dissipated by the heater varies as the square of the voltage. If the heater is designed to provide sufficient heating energy to the battery at the lowest expected input voltage, then when it receives a much higher input voltage the power dissipation may be excessive and may cause the heater to become too hot in which case it may cause irreversible damage to the surrounding material and/or may need to be shut down. Conversely, if the heater is designed to operate safely and without overheating at the highest expected input voltage, then when it receives a much lower input voltage the amount of heat dissipation may be insufficient to adequately and effectively heat the battery.
Thus, there is a need in the art to provide an emergency lighting system, and particularly to LED emergency lighting, wherein an LED lamp is connected to a source of DC current (possibly a combination of DC with a much smaller amplitude AC component added to it) which is able to energize the LED lamp load in the event of an AC power failure. In particular, there is a need to provide a solution for ensuring that a backup power source (e.g., a battery—generally residing inside the emergency lighting system) is not only maintained at a safe and effective operating temperature during charging (i.e., during the time that the emergency lighting system is powered by the AC power), but also that automatically equalizes the battery heating power for different commonly encountered nominal operating voltages (e.g., 120 VAC and 277 VAC in the United States, 230 VAC in Europe, etc). Furthermore, it would be desirable to provide such a solution which can not only ensure that the battery optimally accept charge no matter what the nominal AC voltage is at the particular emergency lighting system installation location, but also can eliminate the need for additional external wires for powering the battery heater. It would also be desirable to provide such a solution which allows the same emergency lighting system to be connected to a charging power source (e.g., AC mains) having any one of a plurality of different nominal operating voltages without the need for flipping or setting an input voltage selection switch to match the nominal operating voltage and/or connecting the charging power source to one or more different input terminals depending on the nominal operating voltage.